This invention relates to arc discharge lamps, and more particularly, to an improved low wattage, high intensity discharge lamp adapted for more efficient starting and operation.
Conventional ballast circuits for operating high intensity discharge lamps have been constructed from leakage reactance transformers and reactors and may or may not include a spike or pulse starter. Typically, the discharge current through the lamp is controlled by the inductive reactance of the transformer core at a 60 Hz line frequency. As will be discussed hereinafter, such ballasts are not particularly suitable for the much smaller size, lower wattage high pressure metal halide arc discharge lamps currently being introduced. For example, one such lamp is discussed in U.S. Pat. No. 4,161,672, which also describes the use of double-ended arc tubes for such lamps. Further, copending applications U.S. Ser. Nos. 132,933 and 132,934, both filed Mar. 24, 1980 and assigned to the present assignee, describe low wattage metal halide arc discharge lamps having a press-sealed single-ended arc tube, that is to say, an arc tube in which both electrodes are located in a press-seal at one end of the arc tube. Practical designs of such lamps have ranged from 100 watts to less than 10 watts. The approximate electrical characteristics of one such lamp, for example, are 50 volts, 1 ampere, 40 watts. In many applications of such discharge lamps, the art tube, which is typically formed of UV-transmitting quartz glass, is enclosed in a glass outer jacket, which provides protection and blocks UV. Further, the outer jacket is pumped to provide a vacuum therein for blocking heat loss from the ignited arc tube and, thus, assure efficient operation.
Considering the aforementioned characteristics of a low wattage lamp, one would ordinarily think that the most desirable ballast to use on 120 volt, 60 Hertz lines would be the simple reactor. The reactor has the advantages of low cost, low loss, small size and weight and good lamp operation. This type of ballast is applicable where line voltage is sufficient to start the lamp. If necessary, a starting device, such as a pulse starter, is often used to facilitate starting such as with high pressure sodium lamps.
Although a low wattage discharge lamp, such as that described in the aforementioned copending application Ser. No. 132,933, ignites under these conditions, great difficulty is exhibited in making the glow-to-arc discharge transition. An obvious way to improve the transition process is to increase the open circuit voltage; for example, increasing the voltage to approximately 240 volts by autotransformer action solves this problem. Such a solution, however, although technically satisfactory, increases the size, cost and especially the losses to a degree that the low wattage high intensity discharge lamp loses its attractiveness as a product.
A copending application U.S. Ser. No. 216,876, filed concurrently herewith and assigned to the present assignee, describes an efficient ballast and starting system that ignites a low wattage discharge lamp, facilitates the transition from glow-to-arc, and operates the lamp satisfactorily. In the specific circuit embodiment disclosed in that application, a lead circuit ballast and a glow-bottle starter device are coupled between a source of AC line voltage and the discharge lamp. More specifically, the glow-bottle starter device is of a unique type having three lead-in wire terminals sealed through an hermetically sealed envelope enclosing a pair of bimetal strips and a rigid conductor rod respectively connected to the three terminal wires. The bimetals are electrically connected together at one end which makes a normally closed contact with the rigid conductor rod in the quiescent state of the device. The ballast circuit comprises an inductive means, such as a reactor choke coil, and a capacitor series connected in that order between a first AC input terminal and one of the bimetal terminals of the starter device. Means are provided for connecting the second AC input terminal to the rigid conductor rod terminal of the starter device, and means are provided for connecting the conductor rod terminal and the other bimetal terminal of the starter device across the terminals of a discharge lamp. Upon initial energization of the circuit, short circuit current through one of the bimetals is operative to flex the bimetals for separating them from the rigid conductor rod to provide an open circuit thereat and a high voltage pulse switching transient across the lamp. Upon starting of the lamp, the lamp current flow through the two bimetals is operative to maintain the bimetal contact separated from the rigid conductor rod. The hermetically sealed envelope of the starter device contains a selected gas at a selected subatmospheric pressure for controlling the amplitude of the high voltage pulse produced by the starter.